PH regulation in symbiotic anemones and corals: A delicate balancing act

Abstract

Symbiotic relationships between photosynthetic and nonphotosynthetic organisms are widespread and underlie a range of ecologically and biogeochemically significant processes. Some of the most well-studied symbiotic relationships are found between the cnidaria (coelenterates) and unicellular dinoflagellate algae (zooxanthellae) such as Symbiodinium spp., and are characteristic of corals, foraminifera, and symbiotic anemones. Indeed, symbiosis is fundamental to the unique biology of globally important coral reef ecosystems (1, 2). It is well known that the presence of zooxanthellae in corals is required for high rates of calcification in the process known as “light-enhanced calcification” (e.g., ref. 3). It is also well known that this symbiosis can be regulated by the cnidarian host and can break down during episodes of coral bleaching in response to environmental stress (e.g., ref. 4). However, the factors that maintain a stable relationship leading to mutual benefits for host and symbiont, including the physiological interactions that underlie energy and nutrient transfer, photosynthesis, and calcification, remain largely unclear. It is critical to know more about these interactions to be able to understand how coral reefs and other major biogeochemical systems may be affected in a rapidly changing environment. The article in this issue of PNAS by Venn et al. (5) sheds new light on the physiological links between the host and symbiont in the symbiotic anemone Anemonia viridis and the coral species Stylophora pistillata. They have adapted fluorescent dye-based pH imaging approaches to examine the influence of the zooxanthellae on host pH. Their findings show that host cytosolic pH can be significantly influenced by the photosynthetic zooxanthellae and should lead to a deeper understanding of the inorganic carbon fluxes that underlie both photosynthesis and calcification.